In a polycrystalline silicon production step using trichlorosilane (HSiCl3) as a raw material, the reaction represented by the following formula mainly progresses to produce polycrystalline silicon according to formula 1.HSiCl3+H2→Si+3HCl  (formula 1)HSiCl3+HCl→SiCl4+H2  (formula 2)
In producing polycrystalline silicon by a Siemens method, trichlorosilane and hydrogen each being a raw material gas are contacted with a guard frame type (inverse u letter type) silicon core wire heated in a reactor to grow polycrystalline silicon on the surface of the silicon core wire through vapor phase epitaxy by a CVD (Chemical Vapor Deposition) method.
Heating the silicon core wire is conducted by electrical heating, and therefore carbon that has high electrical conductivity and heat resistance and is less susceptible to metal pollution is used as a member in the reactor. Examples of the carbon member include a core wire holder for electrifying a silicon cote wire, and a heater for conducting initial heating for electrifying a high-purity, high-resistant silicon core wire.
According to Non Patent Literature 1 (“RECENT CARBON TECHNOLOGY” written by Ishikawa, Toshikatsu and Nagaoki, Toru), it has been understood that carbon reacts with hydrogen at a temperature of 900° C. or higher. A carbon member in a reactor when polycrystalline silicon is deposited is subjected to heat conduction or radiant heat from a silicon core wire, which is being electrically heated, to be at a high temperature, as high as 900° C. or higher, and therefore hydrogen being a supply gas reacts with carbon to produce a slight amount of methane according to the following formula 3.C+2H2→CH4  (formula 3)
In addition to this reaction, a slight amount of methyldichlorosilane having a boiling point close to that of trichlorosilane is contained in trichlorosilane being a raw material gas, and therefore these are decomposed into methane under the reaction conditions of deposition of polycrystalline silicon (see Patent Literature 1(Japanese Patent No. 3727470)).
Accordingly, a slight amount of methane is contained in a reaction exhaust gas exhausted from a polycrystalline silicon production apparatus in addition to tetrachlorosilane, hydrogen, a slight amount of hydrogen chloride (HCl), and unreacted trichlorosilane which are shown in the formula 1 and formula 2.
In addition to these, slight amounts of monochlorosilane (SiH3Cl) and dichlorosilane (SiH2Cl2) are also contained in the reaction exhaust gas as other by-product gases.
It is to be noted that tetrachlorosilane, trichlorosilane, and dichlorosilane will be collectively referred to as trichlorosilanes hereinafter, and the liquids thereof will be collectively referred to as chlorosilane liquids.
Incidentally, in the case where trichlorosilane is converted into silicon by the Siemens method, the conversion rate is low, as low as several to around 10%, and therefore the amount of a raw material gas to be supplied to a reactor have to be large, as large as 15 to 50 Nm3 per 1 kg of deposited silicon. Most of the raw material gas supplied to the reactor is exhausted from the reactor as a reaction exhaust gas, and therefore curbing the loss of the raw material gas becomes necessary to curb production cost. That is, a technique for recovering the reaction exhaust gas to achieve high purification of trichlorosilane and hydrogen, and, on top of that, suppling the trichlorosilane and the hydrogen to a polycrystalline silicon production apparatus again as a raw material gas is essential.
In such a hydrogen gas recovery system, generally, a reaction exhaust gas exhausted from a polycrystalline silicon production apparatus is first separated into hydrogen and the other components with a hydrogen recovery and circulation apparatus directly connected to the polycrystalline silicon production apparatus, and separated hydrogen is then recovered and introduced into the polycrystalline silicon production apparatus again. Examples of such a system are disclosed, for example, in Non Patent Literature 2 (“Report on Outcome of Commission Committed by New Energy Development Organization 1980-1987, Development of Solar Power Generation for Practical Use, Verification of Low Cost Silicon Experiments (development of reduction of chlorosilane by hydrogen), summary version”), Patent Literature 2 (Japanese Patent Laid-Open No. 2008-143775), and Patent Literature 3 (Japanese Patent Laid-open No. 2011-84422) are known.
In such conventional hydrogen gas recovery systems, a method has been adopted in which the chlorosilanes are first condensed and separated, a hydrogen chloride gas is then absorbed and separated with a low-temperature chlorosilane liquid, and the chlorosilanes and hydrogen chloride which are slightly left are finally adsorbed and separated with activated carbon; however, the conventional hydrogen gas recovery systems are not configured to be intended to positively remove methane contained in the exhaust gas.
In adsorbing and separating the chlorosilanes and hydrogen chloride with activated carbon, methane is also adsorbed and removed slightly, but the concentration of methane in a large amount of hydrogen is extremely low, as low as several hundred ppb to several ppm, and therefore the amount of methane to be adsorbed with activated carbon is extremely slight, and most of methane is practically supplied to the polycrystalline silicon production apparatus again without being removed. As a result, when the recovery rate of hydrogen is increased, the concentration of methane in recovered hydrogen is also increased with the increase in the recovery rate.
When such recovered hydrogen is re-supplied to the polycrystalline silicon production apparatus, carbon that is a constitutional element of methane becomes easy to be incorporated in polycrystalline silicon to be deposited (see, for example, Patent Literature 1) to be a big obstacle in producing a high-purity polycrystalline silicon. Accordingly, controlling the concentration of methane in recovered hydrogen is an important problem with the production of high-purity polycrystalline silicon.
From such a viewpoint, as a measure against the generation of methane, surface treatment of a carbon member, and the like have been proposed (see, for example, Patent Literature 4 (Japanese Patent Laid-Open No. 5-213697), Patent Literature 5 (Japanese Patent Laid-Open No. 2009-62252)) for the purpose of suppressing the amount of methane to be generated through reaction of the carbon member and hydrogen in the reactor. However, the surface treatment of a carbon member being a consumable material is very expensive, so that a drastic increase in running cost is brought about, and besides, a slight amount of methane is generated even though the surface treatment is performed on the carbon member, and methane is generated through thermal decomposition of a slight amount of a hydrocarbon compound contained in trichlorosilane being a raw material, and therefore removal of methane in the reaction exhaust gas becomes necessary after all.
In addition, as a measure against the generation of methane due to the decomposition of hydrocarbon-containing impurities such as methyldichlorosilane contained in trichlorosilane, there is a report on control of the concentration of methane passing through an adsorption layer with activated carbon and oh the average pore radius and the like of activated carbon in Patent Literature 1 (Japanese Patent No. 3727470). However, because the concentration of methane in recovered hydrogen is low, as low as several hundred ppb to several ppm, and, on top of that, methane has the characteristic of being unlikely to be adsorbed onto activated carbon, an extremely large activated carbon-filling tank of industrial scale becomes necessary in order to keep the concentration of methane in a large amount of recovered hydrogen 1 ppm or less, so that there is a problem that the cost of facilities increases.
Besides, in Patent Literature 6 (Japanese Patent Laid-Open No. 2010-184831), there is proposed a method in which the retention time of a reaction exhaust gas in a condenser is lengthened to facilitate the dissolution of methane in chlorosilanes being a condensed liquid, thereby reducing the concentration of methane in recovered hydrogen. However, in this method, when the chlorosilanes that have absorbed methane are re-supplied to a reaction apparatus, the dissolved methane is also supplied to the reaction apparatus again.